Pressing silicic acid loaded sheet to improve its chromatographic performance



United States Patent 3,549,486 PRESSING SILICIC ACID LOADED SHEET TO IMPROVE ITS CHROMATOGRAPHIC PERFORMANCE Larry L. Pitt, Florissant, Mo., assignor to Mallinckrodt Chemical Works, St. Louis, Mo., a corporation of Missouri N0 Drawing. Filed July 31, 1967, Ser. No. 657,058 Int. Cl. D2ld 3/00; D2Ih 5/18 US. Cl. 162156 5 Claims ABSTRACT OF THE DISCLOSURE The chromatographic performance capability of sorbent-loaded fiber matrix sheets is improved by pressing the wet sheet between two smooth surfaces at pressures up to 900 p.s.i. The Chromatographic Performance Index (CPI), representing the sum of performance indices with respect to (1) resolution and definition, (2) lateral diifusion and (3) streaking increases from 8 for an unpressed sheet to t] for a pressed sheet, relative to a CPI of for a standard TLC plate.

BACKGROUND OF THE INVENTION This invention relates to sorption and more particularly to sorbent media in sheet form and to methods of preparing such media.

Briefly, the present invention is directed to an improvement in sorbent-loaded sheets, especially sheets of the type which are adapted to use in chromatography.

The preparation of sorbent-loaded sheets is disclosed in Leifield US. Pat. 3,455,818, dated July 15, 1969, for Chromatographic Process. The Leifield patent discloses a sheet comprising a sorbent such as silicic acid supported in a matrix of fibers such as glass micro-fibers. Such sheets are particularly useful for chromatography and may also be used as media for electrophoresis and other separation processes. Leifield discloses their preparation by a process comprising forming a substantially homogeneous aqueous slurry of fibers and the finely powdered sorbent, flowing the resulting slurry onto a porous support surface and removing the liquid therefrom by draining and drying. While Leifields sheets represent a distinct improvement over the sorbent-impregnated chromatographic papers of the prior art their performance does not equal that of a conventional thin layer chromatographic plate with respect to resolution of components, definition (relative area covered by the spot due to a single chemical species) and limitation of streaking and lateral diflusion of spots.

SUMMARY OF THE INVENTION The present invention has as an object the provision of sorbent-loaded sheets having improved chromatographic performance characteristics in comparison with previously known sheets. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the methods hereinafter described, the scope of the invention being indicated in the following claims.

The invention is thus directed to a process for improving the chromatographic performance capability of sorbent-loaded sheets which comprises subjecting the wet sheet to a pressure Within the range of approximately 100 ice p.s.i. to approximately 900 p.s.i. between two smooth surfaces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present process is applicable to sorbent-loaded sheets made with matrices formed from many types of fibers used in conventional or specialized papermaking processes, including natural cellulosic fibers or synthetic fibers such as viscose and acetate rayon, polyamide, polyester, etc. However, where the sheet is to be used in the chromatographic separation of corrosive solutions, and/ or corrosive reagents are to be used in developing the chromatogram, non-cellulosic corrosion-resistant fibers, such as glass, fibrous alumina, asbestos, silica or chemically resistant synthetic polymers such as polytetrafiuoroethylene, should be used.

The most frequently used sorbent is silica (silicic acid). However, other substantially white, inorganic active sorbent powders, such as alumina, zirconia, titania, rare earths, magnesium trisilicate or fullers earth, may be used.

The initial sheet may be laid down by various methods known in the field of paper technology, such as by means of the inclined wire Fourdrinier machine, a cylinder type paper making machine, or hand mold.

While the conventional wet press section of a Fourdrinier machine exerts significant pressure on the newly formed wet web, the primary purpose of this section is to express excess water from the web and not to cause significant compaction of the structure of finished product.

The pressing action contemplated by the present invention must be sufiicient to bring about a substantial compaction of the sheet but not so great as to damage the sheet structure by crushing. It has been found that static pressures in the range of about -900 p.s.i. are necessary to achieve the objects of the present invention. Pressures lower than about 100 p.s.i. ordinarily do not achieve suflicient compaction of the sheet, and pressures substantially above 900 p.s.i., cause significant crushing of the sheet structure.

The objectives of the invention may also be achieved by dynamic pressing, which may be accomplished by adding a special press section to the papermaking machine. The pressure that must be exerted in dynamic pressing is not readily defined, since it will vary with the thickness and composition of the web, with the water content thereof and with the web speed. In general, however, it may be said that in the case of a silica-loaded glassfiber sheet suflicient pressure must be applied to produce a finished sheet having a density not significantly less than 0.35 g./cc.

The pressing action of the present invention may be applied either to the freshly prepared wet sheet or by thoroughly rewetting a sheet prepared as disclosed by the above-noted Leifield application and then pressing the rewet sheet. In either case the pressing operation may be of either the static or the dynamic types.

The following examples further illustrate the invention.

EXAMPLE 1 An aqueous slurry of glass micro-fibers and silicic acid sorbent was prepared. The glass fiber used was Johns- Manville Code lOO fiber (average fiber length, is"; average fiber diameter, 0.05-0.1 micron), supplied by the Johns-Manville Company, 22 E. 40th St. New York, N.Y. The silicic acid sorbent was a TLC sorbent grade of silicic acid sorbent was a TLC sorbent grade of silicic acid supplied by Mallinckrodt Chemical Works under the trademark Silicar. This is a finely powdered (325 mesh) silica hydrogel consisting of approximately 80-90% SiO and 1020% water. The glass fiber (1.5 g.) and sorbent (7.5 g.) were milled with 500 ml. of Water in a high shear mill (Waring-type blender) for five minutes to disperse the solids thoroughly. The resulting slurry was cast into an 8" x 8" Williams hand mold fitted with a fairly fine (50-100 mesh) screen, and the resulting sorbentloaded sheets were allowed to drain.

The drained sheets were couched from the mold, using an absorbent paper to aid in water removal. Five such sheets, including the absorbent paper, were stacked alternately with stainless steel plates in a hydraulic press and pressed at 750 psi. for three minutes. The pressed sheet was dried in circulating air at room temperature.

The final sheet contained 70-75% sorbent by weight; its thickness was 81 mils and its density 0.343 g./cc. Similar sheets produced without pressing were 89 mils thick and had a density of 0.313 g./cc.

EXAMPLE 2 A sorbent-loaded sheet containing approximately 73% luminescent silicic acid sorbent (TLC grade silicic acid containing a small proportion of a commercial cool white alkaline earth halophosphate phosphor) in a glass fiber matrix was formed on an inclined-wire Fourdrinier papermaking machine. The dry sheet had a thickness of 92 mils. and a density of 0.331 g./cc.

The dried sheet was soaked in water for 6090 minutes and pressed and dried as described in Example 1. The pressed sheet had a thickness of 82.5 mils and a density of 0.372 g./cc.

EXAMPLE 3 A sorbent-loaded sheet was prepared generally as described in Example 2, with the exception that 0.20.5% of cationic starch was included, as disclosed in US. Pat. 3,253,978.

The dried sheet from the Fourdrinier had a thickness of 95.5 mils and a density of 0.325 g./cc. This sheet was soaked, pressed, and dried as described in Example 2. The resulting pressed sheet had a thickness of 79.5 mils and a density of 0.3 62 g./cc.

Three performance characteristics of the pressed and unpressed sheets of Example 3 were evaluated. They are typical of similar results obtained with the pressed and unpressed sheets of Examples 1 and 2.

A thin layer chromatographic plate prepared with a 250 micron thick coating of the silicic acid sorbent described in Example 1 was taken as a standard and the performance of this plate in separating Stahls test dye mixture was assigned an arbitrary index value of 5 with respect to each of the following characteristics.

(1) Resolution and definition (2) Lateral difiusion (3) Streaking For each test sheet an index value was assigned for each characteristic, indicating performance relative to the standard.

The sum of the three performance indices for each medium is called the Chromatographic Performance Index and may be considered a semi-quantitative meastire of chromatographic performance of the sorbentloaded sheets in terms of that of the thin layer plate as a standard. Performance data are shown in Table 1.

TABLE 1 [Comparative chromatographic performance in separation of 25 inicrolitor portions of Stahl's test dye lnixturo Performance ii ldox *Stnhls test dye ntixturvA benzene solution containin 0.1" each of butter yellow, sudan rod G and indophenol blue. g m

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process for preparing a sorbent-loaded sheet having improved chromatographic performance characteristics which comprises forming a substantially homogeneous aqueous slurry of fibers and a finely powdered substantially white sorbent, flowing the resulting slurry onto a porous support, producing a wet sheet therefrom and subjecting the wet sheet to a pressure within the range of approximately 100 psi. to approximately 900 p.s.i. between two smooth surfaces.

2. A process as set forth in claim 1 wherein the fibers are non-celluiosic and the resulting sheet has a thickness not greater than approximately 82.5 mils.

3. A process as set forth in claim 2 wherein the sorbent is silicic acid.

4. A process as set forth in claim 2 wherein the fibers are constituted by fibrous glass.

5. A process for preparing a sorbent-loaded sheet having improved chromatographic performance characteristics which comprises forming a substantially homogeneous aqueous slurry of fibers and a finely powdered substantially white sorbent, flowing the resulting slurry onto a porous support, producing a wet sheet therefrom and subjecting the wet sheet to dynamic pressure to produce a finished sheet having a density not substantially less than 0.35 g./cc.

References Cited UNITED STATES PATENTS 2,120,137 6/1938 Mason 162206 2,324,079 5/1938 Greger 210502 2,399,981 8/1941 Britt 162'173 3,455,818 6/1966 Leifield 210502 S. LEON BASHORE, Primary Examiner R. H. ANDERSON, Assistant Examiner US. Cl. X.R. 

